Highly Sensitive Optical Pressure Sensors

The molecular ruby in a solid (red) and dissolved (yellow) state can be used for contactless optical measurement of pressure. (Source: S. Otto, JGU)

Chemists at Johannes Guten­berg Univer­sity Mainz JGU and at the Université de Montréal in Canada have developed a molecular system capable of very precise optical pressure measure­ments. The gemstone ruby served as the source of inspi­ration. However, the system developed by the team headed by Katja Heinze at the JGU Insti­tute of Inorganic Chemistry and Analytical Chemistry and Christian Reber at the Univer­sité de Montréal is a water-soluble molecule, not an insoluble solid. Like the gemstone ruby, this molecule contains the element chromium. This molecular ruby can be used to measure pressure both in the solid state as the gemstone ruby and further­more in solution thanks to its solu­bility. Thus, this molecular system has potential appli­cations in the fields of materials sciences, homo­geneous and hetero­geneous catalysis, and all con­ceivable fields where pressure changes need to be monitored.

Measuring the pressure with the molecular ruby is very straight­forward. The relevant site is irradiated with blue light to be absorbed by the molecular ruby, which then emits infrared radia­tion. Depending on the pressure, the energy of the emitted light varies in a very sensitive manner. The actual pressure can then be read from the lumines­cence energy. The sophis­ticated pressure-dependent lumines­cence measure­ments up to 45,000 bar have been performed by Sven Otto, a doctoral candidate in Heinze’s team, in the labora­tories of the Reber group at the Univer­sité de Montréal.

“The experi­mental work in Montréal was a great experience and the successful proof of concept was just fantastic,” said Otto. “The highest pressures employed in a diamond anvil cell are roughly 45 times higher than that ex­perienced at the deepest known spot in the ocean”, explained Otto. “The very large effects observed with this molecular material are really stunning,” added Christian Reber, an expert in high-pressure lumines­cence spectro­scopy. In fact, the effects are up to twenty times larger with the molecular ruby crystals than with the commonly employed gemstone ruby.

The principle of optical pressure measure­ments using chromium based materials is not new. However, up to now, all these materials are com­pletely insoluble like ruby. Pressure measure­ments with a single type of dissolved molecular species reporting pressure changes directly in solution had not yet been achieved. “However, our molecular ruby can do the trick,” said Katja Heinze. “We hope that our findings will pave the way for completely different appli­cations beyond the clas­sical ones and we are currently working in this direction.” (Source: JGU)

Reference: S. Otto et al.: Molecular ruby under pressure, Ang. Chem., online 2 July 2018; DOI: 10.1002/anie.201806755

Link: Institute of Inorganic and Analytical Chemistry (K. Heinze), Johannes Gutenberg-University, Mainz, Germany

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